objtool: Handle frame pointer related instructions

After commit a0f7085f6a63 ("LoongArch: Add RANDOMIZE_KSTACK_OFFSET
support"), there are three new instructions "addi.d $fp, $sp, 32",
"sub.d $sp, $sp, $t0" and "addi.d $sp, $fp, -32" for the secondary
stack in do_syscall(), then there is a objtool warning "return with
modified stack frame" and no handle_syscall() which is the previous
frame of do_syscall() in the call trace when executing the command
"echo l > /proc/sysrq-trigger".

objdump shows something like this:

0000000000000000 <do_syscall>:
0: 02ff8063 addi.d $sp, $sp, -32
4: 29c04076 st.d $fp, $sp, 16
8: 29c02077 st.d $s0, $sp, 8
c: 29c06061 st.d $ra, $sp, 24
10: 02c08076 addi.d $fp, $sp, 32
...
74: 0011b063 sub.d $sp, $sp, $t0
...
a8: 4c000181 jirl $ra, $t0, 0
...
dc: 02ff82c3 addi.d $sp, $fp, -32
e0: 28c06061 ld.d $ra, $sp, 24
e4: 28c04076 ld.d $fp, $sp, 16
e8: 28c02077 ld.d $s0, $sp, 8
ec: 02c08063 addi.d $sp, $sp, 32
f0: 4c000020 jirl $zero, $ra, 0

The instruction "sub.d $sp, $sp, $t0" changes the stack bottom and the
new stack size is a random value, in order to find the return address of
do_syscall() which is stored in the original stack frame after executing
"jirl $ra, $t0, 0", it should use fp which points to the original stack
top.

At the beginning, the thought is tended to decode the secondary stack
instruction "sub.d $sp, $sp, $t0" and set it as a label, then check this
label for the two frame pointer instructions to change the cfa base and
cfa offset during the period of secondary stack in update_cfi_state().
This is valid for GCC but invalid for Clang due to there are different
secondary stack instructions for ClangBuiltLinux on LoongArch, something
like this:

0000000000000000 <do_syscall>:
...
88: 00119064 sub.d $a0, $sp, $a0
8c: 00150083 or $sp, $a0, $zero
...

Actually, it equals to a single instruction "sub.d $sp, $sp, $a0", but
there is no proper condition to check it as a label like GCC, and so the
beginning thought is not a good way.

Essentially, there are two special frame pointer instructions which are
"addi.d $fp, $sp, imm" and "addi.d $sp, $fp, imm", the first one points
fp to the original stack top and the second one restores the original
stack bottom from fp.

Based on the above analysis, in order to avoid adding an arch-specific
update_cfi_state(), we just add a member "frame_pointer" in the "struct
symbol" as a label to avoid affecting the current normal case, then set
it as true only if there is "addi.d $sp, $fp, imm". The last is to check
this label for the two frame pointer instructions to change the cfa base
and cfa offset in update_cfi_state().

Tested with the following two configs:
(1) CONFIG_RANDOMIZE_KSTACK_OFFSET=y &&
CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT=n
(2) CONFIG_RANDOMIZE_KSTACK_OFFSET=y &&
CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT=y

By the way, there is no effect for x86 with this patch, tested on the
x86 machine with Fedora 40 system.

Cc: [e-mail] # 6.9+
Signed-off-by: Tiezhu Yang <[e-mail]>
Signed-off-by: Huacai Chen <[e-mail]>

tools/objtool/arch/loongarch/decode.c

···
 	switch (inst.reg2i12_format.opcode) {
 	case addid_op:
 		if ((inst.reg2i12_format.rd == CFI_SP) || (inst.reg2i12_format.rj == CFI_SP)) {
-			/* addi.d sp,sp,si12 or addi.d fp,sp,si12 */
+			/* addi.d sp,sp,si12 or addi.d fp,sp,si12 or addi.d sp,fp,si12 */
 			insn->immediate = sign_extend64(inst.reg2i12_format.immediate, 11);
 			ADD_OP(op) {
 				op->src.type = OP_SRC_ADD;
···
 				op->dest.type = OP_DEST_REG;
 				op->dest.reg = inst.reg2i12_format.rd;
 			}
+		}
+		if ((inst.reg2i12_format.rd == CFI_SP) && (inst.reg2i12_format.rj == CFI_FP)) {
+			/* addi.d sp,fp,si12 */
+			struct symbol *func = find_func_containing(insn->sec, insn->offset);
+
+			if (!func)
+				return false;
+
+			func->frame_pointer = true;
 		}
 		break;
 	case ldd_op:

tools/objtool/check.c

···
 				break;
 			}
 
-			if (op->dest.reg == CFI_SP && op->src.reg == CFI_BP) {
+			if (op->dest.reg == CFI_BP && op->src.reg == CFI_SP &&
+			    insn->sym->frame_pointer) {
+				/* addi.d fp,sp,imm on LoongArch */
+				if (cfa->base == CFI_SP && cfa->offset == op->src.offset) {
+					cfa->base = CFI_BP;
+					cfa->offset = 0;
+				}
+				break;
+			}
 
-				/* lea disp(%rbp), %rsp */
-				cfi->stack_size = -(op->src.offset + regs[CFI_BP].offset);
+			if (op->dest.reg == CFI_SP && op->src.reg == CFI_BP) {
+				/* addi.d sp,fp,imm on LoongArch */
+				if (cfa->base == CFI_BP && cfa->offset == 0) {
+					if (insn->sym->frame_pointer) {
+						cfa->base = CFI_SP;
+						cfa->offset = -op->src.offset;
+					}
+				} else {
+					/* lea disp(%rbp), %rsp */
+					cfi->stack_size = -(op->src.offset + regs[CFI_BP].offset);
+				}
 				break;
 			}
 

tools/objtool/include/objtool/elf.h

···
 	u8 warned	     : 1;
 	u8 embedded_insn     : 1;
 	u8 local_label       : 1;
+	u8 frame_pointer     : 1;
 	struct list_head pv_target;
 	struct reloc *relocs;
 };